In internal combustion engines of the type in which fuel is directly injected into each cylinder, there has been known a fuel injection control system which is provided with a low pressure fuel pump that serves to draw up fuel from a fuel tank, and a high pressure fuel pump that serves to cause the fuel thus drawn up by the low pressure fuel pump to rise up to a pressure at which the fuel can be injected into each cylinder.
In the fuel injection control system as mentioned above, in order to suppress the energy consumption accompanying the operation of the low pressure fuel pump, it is desired to decrease the delivery pressure (feed pressure) of the low pressure fuel pump as much as possible.
In a first patent document, there is described a technique in which in a system to regulate the delivery pressure of a high pressure fuel pump by means of a pre-controlled amount as well as an amount of open control and an amount of closed loop control, the delivery pressure of a low pressure fuel pump is caused to decrease, in cases where an output of an integrator, to which the amount of open control and the amount of closed loop control are supplied, becomes zero.
In a second patent document, there is described a technique of adjusting the delivery pressure of a low pressure fuel pump in accordance with an amount of driving of a pressure control valve or a relief valve of a high pressure fuel pump.
In a third patent document, there is described a technique in which in cases where the driving duty of a high pressure fuel pump becomes equal to or greater than a predetermined value, a determination is made that vapor has been generated, thus causing feed pressure to go up.